The attaching and effacing (A/E) pathogens enteropathogenic Escherichia coli (EPEC) and enterohemorrhagic E. coli (EHEC) are major human pathogens, and the diseases they effect cause a significant public health risk. The related Citrobacter rodentium is a natural mouse pathogen that is used as an in vivo model for A/E lesion forming pathogens, including EPEC and EHEC. The major aim of this fellowship application is to address how C. rodentium adapts its metabolism to exploit the metabolite environment provided by the resident microbiota to promote successful colonization. The specific aims are: 1. The hypothesis that C. rodentium can respond to its nutritional environment by modulating its virulence will be tested. This will be accomplished by screening for the induction of virulence genes with a luciferase reporter strain of C. rodentium after exposure to hundreds of unique nutrients on Biolog Phenotype MicroArray plates. The influence of disaccharides and fatty acids upon virulence gene induction will also be tested. Finally, mutations will be made in critical genes involved in the catabolism and transport of the nutrients found to affect virulence gene induction, and then tested for their ability to induce virulence and colonize the host. 2. The route of infection with C. rodentium begins by ingestion, followed by early colonization of the cecum and subsequent colonization of the colon. The hypothesis that the metabolites provided by the resident microbiota influence susceptibility to C. rodentium colonization in the cecum will be tested. The composition of the microbiota will be shifted with antibiotics and then tested for susceptibility to colonization. The dominant nutrients present in the cecum and small bowel of these mice will be identified by metabolomics and fatty acid analysis, and directly tested for their influence upon colonization.